The present invention relates to a so-called strapping head to be used in packaging machines and more particularly relates to the strap driving system of the strapping head, which is designed to ensure a high operating speed, albeit using simple means which can be easily maintained and are less costly.
Some examples of strapping machines are described in the patents IT-B-1,135,722 dated Mar. 24, 1981, EP-C-0,603,868 dated Nov. 22, 1993xe2x80x94both of which are in the name of the same Applicantxe2x80x94and U.S. Pat. No. 5,379,576 dated Jan. 10, 1995 in the name of STRAPACK Corp., to which reference is made here in order to illustrate the invention more clearly.
As is known, in these strapping machines, strap driving means are provided, said means performing essentially three functions: a) firstly, drawing the strap from a reel and inserting it into a special guide track for guiding it around the product to be packages; b) then, when the strap has travelled around the whole guide track until its front terminal part stops into a first fixing gripper, recovery of the said strap until it is wound around the product to be packages; c) finally, tensioning of the strap, after completion of which the rear terminal part of the strap is likewise blocked by a second clamping gripper.
In addition to said driving means, a strapping head comprises mainly means for joining together, by means of hot clamping one onto the other, the two fixed ends of the strap; after which, cutting of the strap and conveying away of the packaged product are performed.
The present invention relates to a head intended mainly for tying small parcels, where strapping is performed with a relatively thin strap and the tightening force is obtained by pulling the strap over a predetermined distance. The patent EP-B-0,603,868 refers, on the contrary, to a head in which the tightening force is determined depending on a predefined load which is precisely determined and kept uniform during each strapping operation. This machine was therefor a relatively complex structure; it uses a stronger strap and achieves a high tightening force by means of two drive rollers around which the strap is would over a wide arc.
In the arrangement according to the patent U.S. Pat. No. 5,379,576xe2x80x94which is regarded as being the prior art closest to the invention, on which the preamble of claim 1 is basedxe2x80x94three strap drive wheels are provided for driving the strap, i.e., as described more fully below, an insertion wheel and a recovery wheel as well as a tensioning wheel located between the first two. Each of these strap drive wheels co-operates with a respective pressure roller so that the strap is driven only when the pressure roller pushes it against the surface of the respective drive wheel, even if the latter is continuously rotating. In fact, the pressure rollers are each mounted idle on their respective axis and the latter is able to assume two positions, i.e. a working position, where it causes its roller to press the strap against the surface of the respective strap drive wheel, and a rest position, where it positions its roller at a short distance from the respective strap drive wheels, such that the latter is able to rotate freely without driving the strap. Displacement of the pressure roller axes is performed, for example, by means of actuation of corresponding electromagnets.
In the strapping machine according to U.S. Pat. No. 5,379,576, the strap passes firstly between the insertion wheel and the associated pressure roller; it then advances along a first guide track as far as the tensioning wheel; it then passes around the latter, over a wide angular segment; it then passes into a second track guide as far as the recovery wheel; and finally from here, via a third guide track, it advances until it is wound around the product to be packaged. By means of a single actuating motor which is always running, three movement transmission systems, generally consisting of drive belts, ensure driving of, respectively: a) the insertion wheel and the recovery wheel at high-speed; b) a reduction gear, which drives the tensioning wheel at low-speed; and c) a reduction gear which drives a shaft which carries a series of cams for performing the various functions of the machine. Engagement and disengagement means allow switching of the actuation of the different transmissions so as to perform, respectively, insertion and recovery or tensioning or the other functions.
During insertion and recovery operations, said first high-speed movement transmission is active; switching from one operation mode to the other is performed by simply activating the pressure roller associated with the insertion wheel or else the pressure roller associated with the recovery wheel; during the tensioning operation, an electromagnetic clutch engages the second movement transmission with the low-speed reduction gear, with simultaneous activation of the respective pressure roller.
The strapping machine according to U.S. Pat. No. 5,379,576, however, has various drawbacks: first of all, driving of the tensioning wheel by means of said low-speed transmission, with the associated electromagnetic engagement means, involves a costly constructional design and moreover is subject to incorrect adjustment as a result of wear during use. Moreover, the said use of a second movement transmission is the source of increased costs. Finally, during high-performance operationxe2x80x94where up to 40-50 strapping cycles per minute are performed, as required in modern machinesxe2x80x94the start-up and stoppage of an additional movement transmission has an entirely negative effect on the machine cycle time.
The object of the present invention is therefore to provide a strapping machine which has a simpler and less costly structure, but is equally efficient at high working performances and which overcomes the drawbacks mentioned. This object is achieved, in a machine of the type illustrated in U.S. Pat. No. 5,379,576, by means of the characteristic features indicated in claim 1.
Moreover, although the machine according to the invention is based on a tightening action determined on the basis of a predetermined tightening distance, instead of a given tightening load, it allows higher and much more uniform tightening forces to be exerted.